The heating and working up operations of milk for making a cappuccino, i.e. a drink containing coffee and worked up milk, are generally carried out by conveying a jet of steam into a vessel, for example a jug, containing cold milk through a nozzle at the end of a steam pipe, generally equipped with a shut-off valve for controlling the flow of steam.
Devices for heating and working up milk, of the type mentioned above, are known in the state of the art in that they offer the advantage of being able to work up milk with a desired amount of froth and temperature.
Indeed, since the heating and working up is entrusted to the operator, he shuts off the supply of the flow of steam once the desired temperature is reached and after having admitted enough air to form a creamy and compact froth.
However, such operations are somewhat complex and difficult to calibrate since to obtain the desired froth at the desired temperature it is necessary to carry out actions which require a specialised ability in the field.
Consequently, the task of such operations is generally reserved to expert operators, usually with a certain skill achieved over time.
Indeed, to obtain a very hot drink with a very fine and compact froth it is necessary to work the milk with suitable movements made to the vessel, in other words a correct positioning of the steam pipe with respect to the vessel and an appropriate frequency and length of movements up and down of the vessel itself.
In particular, the operator must be capable of working up a sufficient amount of frothed milk before it exceeds the desired temperature and in any case before the bubbles start to burst due to the expansion of the gas trapped inside the milk, under the effect of the temperature.
The drawback is all the more serious if the nozzle is insufficiently submerged in the milk, since the steam discharged through the nozzle would produce dangerous jets without obtaining the formation of froth; on the other hand, a prolonged submersion of the nozzle in the milk would cause its excessive heating without entrapping air. Moreover, in the case of the evaporation temperature being exceeded, the air bubbles formed previously would expand to then burst making the froth look much less pleasant and compact.
The task is made even more difficult when espresso machines for the home are used which have a lower steam flow rate in which, therefore, the probability of obtaining a desired working up and a sufficient amount of compact froth is generally low.
According to a first solution of the prior art and disclosed in EP-A-0195750, the working up of milk is obtained through a pipe for supplying the steam in communication with a steam generator and open near to an intake chamber from which extend a supply pipe for the milk and a pipe for supplying the air from the outside, this intake chamber being put in communication with a mixing chamber from which the worked up milk comes out. The intake is carried out thanks to the Venturi effect which is possible in virtue of the special shape given to the intake zone.
This solution involves a few drawbacks and disadvantages, both from the point of view of the froth obtained, as well as from the point of view of choosing the temperature.
Indeed, the temperature is directly proportional to the steam flow rate and milk flow rate, said milk flow rate depending upon the depression generated in the intake chamber which in turn depends upon the amount of air injected to form the froth.
A second solution of the prior art foresees the use, as disclosed in DE-4445436, of a device for producing froth from milk to make a cappuccino equipped with a first pipe for supplying steam fed from a steam generator, a second pipe for supplying air in communication with a compressed air generator, a third pipe for supplying milk and a mixing zone, in which the fluids brought by the supply pipes come together before going into the frothing chamber, from which the worked up milk comes out.
Even this solution involves a few drawbacks and disadvantages.
Indeed, the demand for worked up milk with characteristics of specific froth consistency and temperature cannot be satisfied by such a device since the flow rate of sucked in milk depends upon the depression generated in the mixing chamber, and the temperature depends upon both the steam flow rate and the milk flow rate. Consequently, the limited versatility of such a device does not allow milk froth with a particular consistency required by the most demanding customers to be produced.
Moreover, with devices comprising a pipe for supplying milk, it may be so that after long periods of time unused, in the mixing zone milk stagnation forms, with harmful consequences not just for the smell of the milk to be subsequently frothed but also for hygienic reasons, since this causes the growth of undesired microorganisms. Therefore, it is necessary to periodically clean the pipe for supplying the milk, such cleaning being difficult since it requires disassembly operations and often the use of appropriate disinfectants.
According to a further solution of the prior art, thermocouples coupled with the steam nozzle allow the jet to be stopped automatically when the desired temperature level is reached.
Nevertheless, this solution also suffers the aforementioned drawbacks and in particular does not allow froth to be formed without a skilled operator capable of obtaining it manually.
Moreover, for all of the devices described above, the types of automation adopted have great restrictions in terms of the consistency and amount of froth which can be obtained from the working up of milk, as well as the temperature of the frothed milk, and in any case they do not allow the user to operate the device itself manually, if so desired.